fix(positioning): make tray snapping DPI independent

[CodeZeno]

Summary

• Make tray-relative widget snapping independent of display DPI so widgets retain the expected position across scaling configurations.

Testing

• Updated positioning logic and regression coverage for DPI-aware tray snapping.
• Full test suite not run.

Checklist

• I have read CONTRIBUTING.md and CODE_OF_CONDUCT.md.
• The change is focused and contains no credentials or sensitive data.
• I have added or updated relevant documentation and tests, where applicable.
• I have left changelog and version changes to maintainers unless requested.